Olefinic hydrocarbons are feedstocks for a variety of commercially important additional reactions to yield fuels, polymers, oxygenates and other chemical products. The specific olefin isomer, considering the position of the double bond or the degree of branching of the hydrocarbon, may be important to the efficiency of the chemical reaction or to the properties of the product. The distribution of isomers in a mixture of olefinic hydrocarbons is rarely optimum for a specific application. It is often desirable to isomerize olefins to increase the output of the desired isomer.
Butenes are among the most useful of the olefinic hydrocarbons having more than one isomer. A high-octane gasoline component is produced from a mixture of butenes in many petroleum refineries principally by alkylation with isobutane; 2-butenes (cis- and trans-) generally are the most desirable isomers for this application. Secondary-butyl alcohol and methylethyl ketone, as well as butadiene, are other important derivatives of 2-butenes. Demand for 1-butene has been growing rapidly based on its use as a co-monomer for linear low-density polyethylene and as a monomer in polybutene production. Isobutene finds application in such products as methyl methacrylate, polyisobutene and butyl rubber. The most important derivative influencing isobutene demand and butene isomer requirements, however, is methyl t-butyl ether (MTBE) which is experiencing rapid growth in demand as a gasoline component. Pentenes also are valuable olefinic feedstocks for fuel and chemical products.
Catalytic isomerization to alter the ratio of isomers is one solution to this need. Since ethers must be supplied at lower cost to find widespread use as a fuel product and since isomerization competes with increased feedstock processing as a source of desired isomers, an isomerization process must be efficient and relatively inexpensive. In one aspect, a catalytic isomerization process must recognize olefin reactivity: isobutene in particular readily forms oligomers which could require a reconversion step to yield monomer if produced in excess. The principal problem facing workers in the art, therefore, is to isomerize olefins to increase the concentration of the desired isomer while minimizing product losses to heavier or lighter products.
U.S. Pat. No. 4,797,133 discloses a process for the recovery of butene-1 from a mixed C.sub.4 feedstream which also contains isobutylene, butene-2, isobutane, and normal butane. The C.sub.4 feedstream is passed through an etherification zone to selectively convert isobutylene to an ether to produce a first stream comprising the product ether and C.sub.4 hydrocarbons and a second stream comprising isobutane and butene-1. The second stream is then separated to yield butene-1. The first stream is alkylated for use in a motor fuel. Butene-2 isomers (cis- and trans-) are desired feedstocks to an alkylation reactor in which the butene-2 isomers react with isobutane to produce alkylate.
U.S. Pat. No. 4,423,264 discloses a process for the production of a pure butene-1 and a premium gasoline from a C.sub.4 olefinic hydrocarbon fraction. According to the disclosure, the C.sub.4 olefinic hydrocarbon fraction is polymerized and disproportionated to partially convert the isobutene to a gasoline/jet fuel boiling range component (e.g., an isobutene dimer and trimer), hydrogenating the gasoline/jet fuel boiling range fraction to produce a stabilized fuel component, and fractionating the remaining C.sub.4 olefins to obtain a high purity butene-1 product.
U.S. Pat. No. 5,523,502 discloses a process for the deep catalytic cracking of petroleum feedstocks to produce a full range of synthetic hydrocarbons including a C.sub.4 hydrocarbon fraction which includes C.sub.4 paraffins and butenes. In processing the C.sub.4 hydrocarbon fraction, combinations of separate processing include: hydroisomerization of butene-1 to butene-2, butadiene hydrogenation, and etherification. The remaining butenes are passed to an extractive distillation process to separate the olefins (butene-1 and butene-2) from any paraffins (normal butane, etc.), and the olefins are passed to a skeletal isomerization unit and therein converted to isobutene which is recycled to the etherification zone to produce more MTBE.
Processes for the isomerization of olefinic hydrocarbons are widely known in the art. Many of these use catalysts comprising phosphate. U.S. Pat. No. 2,537,283 (Schaad), for example, teaches an isomerization process using an ammonium phosphate catalyst and discloses examples of butene and pentene isomerization. U.S. Pat. No. 3,211,801 (Holm et al.) discloses a method of preparing a catalyst comprising precipitated aluminum phosphate within a silica gel network and the use of this catalyst in the isomerization of butene-1 to butene-2. U.S. Pat. Nos. 3,270,085 and 3,327,014 (Noddings et al.) teach an olefin isomerization process using a chromium-nickel phosphate catalyst, effective for isomerizing 1-butene and higher alpha-olefins. U.S. Pat. No. 3,304,343 (Mitsutani) reveals a process for double-bond transfer based on a catalyst of solid phosphoric acid on silica, and demonstrates effective results in isomerizing 1-butene to 2-butenes. U.S. Pat. No. 3,448,164 (Holm et al.) teaches skeletal isomerization of olefins to yield branched isomers using a catalyst containing aluminum phosphate and titanium compounds. U.S. Pat. No. 4,593,146 teaches isomerization of an aliphatic olefin, preferably 1-butene, with a catalyst consisting essentially of chromium and amorphous aluminum phosphate. None of the above references disclose the olefin-isomerization process using the non-zeolitic molecular sieve (NZMS).
The art also contains references to the related use of zeolitic molecular sieves. U.S. Pat. No. 3,723,564 (Tidwell et al.) teaches the isomerization of 1-butene to 2-butene using a zeolitic molecular sieve. U.S. Pat. No. 3,751,502 (Hayes et al.) discloses the isomerization of mono-olefins based on a catalyst comprising crystalline aluminosilicate in an alumina carrier with platinum-group and Group IV-A metallic components. U.S. Pat. No. 3,800,003 (Sobel) discloses the employment of a zeolite catalyst for butene isomerization. U.S. Pat. No. 3,972,832 (Butler et al.) teaches the use of a phosphorus-containing zeolite for butene conversion in which the phosphorus has not been substituted for silicon or aluminum in the zeolite framework. U.S. Pat. No. 5,292,984 discloses the use of a non-zeolitic molecular sieve, NZMS, for the isomerization of pentenes in a pentene-containing feedstock comprising a raffinate from an etherification process. U.S. Pat. No. 5,292,984, which is hereby incorporated by reference, discloses the use of a catalyst comprising at least one NZMS and having the absence of a platinum-group metal demonstrates surprising efficiency in converting butene-2 to isobutene or butene-1 in a butene isomerization operation and in the skeletal isomerization of pentenes.
Efficient production of butene-1 has remained a problem in the art, requiring complex, multi-step processes to recover butene-1, often as a by-product of motor fuel production. In such processes, the objective is butene-2 which can be alkylated to produce a high octane, low vapor pressure product. Often when butene-1 is isolated, it is further skeletally isomerized to produce more isobutene.
It is the objective of the present invention to provide a simplified process for the recovery of butene-1 from C.sub.4 olefin streams. It is a further objective of the present invention to provide a reduced cost process for selectively and directly producing butene-1 from C.sub.4 olefinic hydrocarbon streams.